The present invention relates to a pump including an impeller having a magnet or a second conductor therein, which is adapted to be directly rotated by virtue of magnetic fields of stator coils, and more particularly to a pump including an impeller which is rotated within a casing in close proximity thereto via ceramic bearing members, one bearing member having spiral grooves on its slide surface.
In a conventional pump, an impeller having a magnet or a secondary conductor therein is directly driven by stator coils, as disclosed in the Japanese Patent Laid-Open Specification No. 49-129106.
FIG. 1 is a longitudinal cross sectional view of such a conventional pump. In the drawing, an impeller 13 arranged within a pump casing 6 is provided with an annular permanent magnet 16 in its outer rear end perpendicular to a rotary axle 12 of the impeller 13, and the surface of the annular permanent magnet 16 is coated with a synthetic resin (not shown) for preventing it from contacting with liquid in the casing 6. The rear end of the casing 6 is closed by a rear plate 20 on which stator coils 23 and a rear bearing 24 for supporting the impeller 13 are mounted. A- front bearing 25 for supporting the impeller 13 is mounted to the casing 6. The stator coils 23 are covered by a plastic film 27 for preventing them from contacting with the liquid.
When the impeller 13 stands still, the impeller 13 is biased towards the rear plate 20 by the drawing force of the permanent magnet 16, and, when the impeller 13 is rotated, in general, the impeller 13 is urged frontwards of the pump casing 6 by its discharging pressure. Hence, the bearings 24 and 25 are constructed so as to support the radial load as well as the thrust load. The impeller 13 shown in FIG. 1 is a so-called open type having a suitable structure for dealing with liquid containing solid particles such as crystals.
In the conventional pump of FIG. 1, a secondary conductor may be provided in the impeller 13 instead of the permanent magnet 16. In this case, the stator coils 23 are so wound as to impart rotary magnetic fields when an alternating current is applied to the stator coils 23.
In FIG. 2, there is shown another conventional pump. In this embodiment, a motor 1 has a rotary shaft 2 therein, and an iron yoke 3 is secured to the free end of the rotary shaft 2. An annular permanent magnet 4 is attached to the iron yoke 3, and the N and S polarities are alternately magnetized at equal intervals in the peripheral direction of the annular permanent magnet 4. A lower bracket 5 of the motor 1 is mounted to a pump casing 6 through a non-magnetic partition plate 7 by bolts and nuts 8. A plurality of arm members 10 are radially arranged in a suction opening 9 of the pump casing 6, and suction paths are defined by the arm members 10. A boss 11 is disposed to the center of the arm members 10, and a rotary pump shaft 12 which is coaxial with the rotary shaft 2 of the motor 1, is mounted to the boss 11. A bearing member 14 having flanges, press-fitted in a central hole of an impeller 13, is fitted on the rotary pump shaft 12. A liner ring 15 is press-fitted in the suction opening portion of the pump casing 6 in order to seal the gap between the outer periphery of the suction inlet of the impeller 13 and the inner surface of the pump casing 6.
In the upper end of the impeller 13, permanent magnets 16 of different N and S polarities are alternately and radially arranged, opposing the annular permanent magnet 4 of the motor 1 through the partition plate 7.
When the motor 1 is driven, the rotary shaft 2 is rotated together with the yoke 3 and the annular permanent magnet 4, and then the permanent magnets 16 of the impeller 13 are driven by the magnetic force of the annular permanent magnet 4, thereby rotating the impeller 13 around the pump shaft 12. Accordingly, the liquid is sucked from the suction opening 9 of the pump casing 6 and then is sucked up by the impeller 13, and the pressurized liquid is discharged from an outlet 21 of the pump casing 6. A part of the pressurized liquid flows between the upper part of the impeller 13 and the partition plate 7 towards the central portion to pass through balancing holes 22 and is then returned to the low pressure side of the impeller 13, for balancing the pump thrust.
In the above described pump, the rotary shaft 2 of the motor 1 is separated from the rotary pump shaft 12, and hence no special sealing means for the shaft is necessary, and the liquid in the pump side is separated from the motor side by the partition plate 7. Accordingly, a strong acidic liquid, a strong alkaline liquid, a strong toxic liquid or a liquid to be prevented from leaking outside the pump may be transferred by the pump of this kind, and further, in turn, by using this pump, a liquid to be dealt with may be prevented from having a contaminant from the outside mixed therewith in a producing process of foods, medicines, soft drinks, semi-conductors and so forth.
However, in fact, in the conventional pumps shown in FIGS. 1 and 2, the impeller is supported by the bearings in contact therewith, and thus, when the impeller is rotated, powder is produced from the wear of the bearing portions. In the conventional pump of FIG. 2, the magnetic gap between the annular permanent magnet 4 of the driving side and the permanent magnets 16 of the impeller 13 is increased, and accordingly a pump having a large output cannot be obtained. Further, in these conventional pumps, when stopping the pumps for a long time, the material of the bearings is subjected to a plastic deformation by the drawing force of the permanent magnets, and, when the pump is driven again, it is impossible to operate the pump or a large starting torque is necessary to start the pump. Further, in the conventional pumps, since the rotary shaft of the impeller is supported by the bearing means or the impeller is rotatably supported at its axle by the bearing means, that is, the conventional impeller requires a rotary shaft having a certain length along the axis of the impeller, the length of the impeller and hence of the pump in the axial direction is restricted and can not be reduced.